258 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



on 



Iv concerned now with one element of the ocean, the sodium, and 



it will be eas}^ to .show that complete neutralization of the acid hydro- 

 sphere would have been attended by only a relatively small introduc- 

 tion of sodium into the ocean. 



We can make a rough estimate of the results of this primeval chem- 

 ical denudation — and hence of the correction on the estimate of 

 geological time involved in the primitive saltness of the ocean — by 

 allocating the action of the acid among the constituents of the early 

 crust; but we have iirst to inquire into the percentage relations of 

 these constituents. 



Mr. F. W. Clarke has estimated the percentage amounts of the ele- 

 ments contained in the earth's surface crust. In Mr. Clarke's first 

 report ^ the mean of 880 selected analyses of American and fLuropean 

 igneous, volcanic, and crystalline rocks is tabulated along with the 

 means of the component analyses divided into local groups, as the 

 rocks of the Western States, of northern California, of European 

 volcanic and crystalline rock. etc. ; and it is remarked as the result of 

 comparing these groups that "the thesis that the crust of the earth is 

 fairly homogeneous in composition is thus sustained by positive evi- 

 dence.'" In a later publication"^ 960 analyses are consulted, and these 

 of a still more carefully selected and reliable character, giving an 

 average "which may fairly represent the composition of the older 

 crust of the earth." The result, which closely agrees with the earlier 

 estimate, is contained in the column below. 



SiOa 59. 77 



AUG, 15. 38 



Fefi; 2. 65 



FeO 3. 44 



CaO 4. 81 



MgO 4. 40 



K^O 2.83 



Na^O 3.61 



H,0 1.51 



TiO, 53 



PA 21 



99.14 



This approximates to a diorite, and would fall among Rosenbusch's 

 seriesof "granito-dioriti.schen" and "gabbro-peridotitischen" magmas.' 



Such a rock or lava attacked by a heated solution of hydrochloric 

 acid must ultimately yield its iron, calcium, magnesium, potash, and 

 soda as chlorides. 



1 Bulletin of the U. S. Geological Survey, No. 78, 1891, p. 34. 



2 Bulletin of the U. S. Geological Survey, No. 148, 1897, p. 12. 



='Elemente der Gesteinslehre. Stuttgart, 1898, p. 187. See No. 15 of this group 

 for a rough approximation to Clarke's average. 



